Evaporatively cooling a fuel cell stack is disclosed in Publication US 2006/0141330 A1. In contrast with sensible cooling management systems, in which heat generated in the stack is conveyed either to circulating water passing through the cells or to coolant passing through coolant plates, heat is removed by evaporating the water into the air stream. Benefits of evaporative cooling include a much lower inventory of coolant (typically water), fewer water carrying auxiliary components external to the stack, and a more compact cell design with shallower coolant and air channels.
However, polymer electrolyte membrane (PEM) fuel cells that are evaporatively cooled have lower membrane durability because of higher operating temperatures. In sensibly cooled fuel cell stacks, operation at all current densities with air utilization greater than about 60%, for example, maintains the cell stack in water balance (that is, the amount of water exiting with the air stream is equal to the amount of water which is produced by the cells). The relatively high air utilization allows the gas to exit the cells at a temperature of approximately 65° C. (about 150° F.) at ambient exit pressure. On the other hand, an evaporatively cooled fuel cell stack operating around 60% air utilization will have an air exit temperature of approximately 85° C., increasing slightly as a function of current density.